Method for detecting entry of a low hydrogen content gas into borehole formations

ABSTRACT

The method of detecting the point of entry of a gas into a producing well, which comprises the steps of first measuring the hydrogen content of the hydrocarbon products native to the formations in correlation with depth, then injecting a gas having less hydrogen content than said hydrocarbon products into the formations surrounding the well, then measuring the hydrogen content of the hydrocarbon products within the well and surrounding formations in correlation with depth after said gas has been injected, and comparing said first and second named measurements to determine the location of the point of entry of said gas into said well as indicated by a reduction in the hydrogen content of said second measurement relative to said first measurement at the same depth.

United States Patent [191 Hartsell METHOD FOR DETECTING ENTRY OF A LOWHYDROGEN CONTENT GAS INTO BOREHOLE FORMATIONS Inventor: Kenneth D.Hartsell, Big Lake, Tex.

Assignee: The Western Company of North America, Fort Worth, Tex.

Filed: July 26, 1973 Appl. No.: 382,857

References Cited UNITED STATES PATENTS 9/1961 Armstrong 250/259 Xl2/l97l Jorden, Jr. 250/259 7/1973 Murphy 250/259 Sept. 23, 1975 PrimaryExaminer-Archic R. Borchelt Attorney, Agent, or Firm-Mason, Kolehmainen.Rathburn & Wyss [57] ABSTRACT The method of detecting the point of entryof a gas into a producing well, which comprises the steps of firstmeasuring the hydrogen content of the hydrocarbon products native to theformations in correlation with depth, then injecting a gas having lesshydrogen content than said hydrocarbon products into the formationssurrounding the well, then measuring the hydrogen content of thehydrocarbon products within the well and surrounding formations incorrelation with depth after said gas has been injected, and comparingsaid first and second named measurements to determine the location ofthe point of entry of said gas into said well as indicated by areduction in the hydrogen content of said second measurement relative tosaid first measurement at the same depth.

6 Claims, 3 Drawing Figures SOURCE 7 v v r DETECTING SYSTEM US PatentSept. 23,1975 3,908,122

36 3 FIG. 2 5 ..J

3 RATE AMPLIFIER HL 30 FIG 1 32 RECORDER 42 44 J Li? DETECTING SYSTEMHYDROGEN 26 ii DECREASES g NEUTRON z SOURCE 1 5 2 Z w :'i t 54 j DEPTH50;

E: "-52 GAS i MOVEMENT\\ FIG. 3

METHOD FOR DETECTING ENTRY OF A LOW HYDROGEN CONTENT GAS INTO BOREHOLEFORMATIONS The present invention relates to secondary recoveryoperations for the recovery of oil from oil wells which have becomenaturally depleted or to increase the production from oil wells whoseefficiency is low under normal depletion conditions. Specifically, thepresent invention relates to an improved method for detecting the entryofa low hydrogen content gas into the formations adjacent a producingwell.

In the secondary recovery field of oil well operations, a number ofmethods have become popular to enhance the recovery of oil from aflowing well whose productivity is low. These methods include waterflooding, gas injection, thermal methods and miscible-phase flooding. inwater flooding and gas injection methods, a fluid such as gas or wateris forced into the producing formation by means of a centrally locatedinjection or inlet well and the injected fluid tends to flush the oilfrom the rock toward the surrounding producing wells. While the leadingedge of the injected fluid tends to advance in a circular pattern duringthe initial stages of the injection operation, during later stages thepattern becomes irregular and eventually the injected fluid will reachone of the producing wells at a projection point of the pattern whilesome areas of the reservoir remain full of oil. This breakthrough of theinjected fluid into the producing well must then be detected and asuitable cementing, packing or flow control operation must be performedat the point of entry of the injected fluid so as to seal off thisescape point for the injected fluid and permit further invasion of theinjected fluid into the re-. maining uninjected areas. I i

When gas is employed asthe injection fluid two sur vey type ofmeasurements arenormally employed to detect the entry of gas into one ofthe producing or ing to another method, thechange in densityofthe ormore of the disadvantages of the prior art arrangements discussed above.

It is a further object of the presentinvention to provide a new andimproved method for accurately detecting the breakthrough or entry pointof an injected gas into a flowing well which is suitable for use insituations where the injected gas is of relatively low hydrogen content.

Briefly considered the method of the present invention comprises thesteps of first measuring the hydrogen content of the hydrocarbonproducts within the well in correlation with depth. This may beconveniently done by running a neutron log in the well at the time it isdrilled or-before initiating the secondary recovery operation.

A gas having less hydrogen content than the petroleum products in theformations is then injected into the formations surrounding theproducing well, this being customarily done by means ofa centrallylocated injection well. When a breakthrough of the injected gas into thearea of the producing well occurs, as determined by inspection of thegas content of the flowing well at the surface, the point of entry ofthis breakthrough may then be located by again measuring the hydrogencontent of the hydrocarbon products within the well and surroundingformations in correlation with depth and comparing the first and secondmeasurements to determine the location of the point of entry. of theinjected gas into the formations as indicated by a reduction in thehydrogen content of the second measurement relative to the firstmeasurement at the same depth.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification taken inconnection with the the present invention may be employed;

FIG, '2 is a sectional-view ofa neutron logging system fluid in theborehole is measured as the injected gas;

whichhas broken through the formations surrounding the borehole at aparticular point, mixes with the borehole fluid. Both of these methodsfor detecting gas entry require that the injected gas must be miscibleproducing wells because their entry into the borehole is not restrictedby the flowing oil and they permeate the oil column easily so that theycan be readily measured by the above two methods. However. when theinjected gas used in the secondary recovery operation is a low hydrogencontent gas. such as carbon dioxide, the breakthroughs in producingwells are difficult to locate. This is due to the resistance of the oilcolumn within the borehole to accept the gas, thus restricting theexpansion of the gas which would cause temperature changes capable ofdetection and diminishing the mixing of the injected gas with the fluidcolumn to produce a detectable change in density.

It is a principal object therefore of the present invention to provide anew and improved method for dctecting the entry ofa low hydrogen contentgas into the formations adjacent the flowing well which eliminates onewhich may be employed in performing the entry detection method of thepresent invention; and

FIG. 3 is .a composite chart of the two neutron logs made before andafter entry of the injected gas, and illustrating the manner in whichthe exact point of entry may be accurately determined. Referring now tothe drawings, a typical secondary recovery operation-jisshowndiagrammatically in FIG.

.1. wherein a centrallylocated injection well 10 is positioned withinasurrounding group of producing wells 12. A fluid'suchas gas or water isinjected into the well 12 under pressure and spreads out into thesurrounding formations so that it tends to flush the oil from the rocktoward the producing wellslZ. During the initial stages of the injectionoperation .the leading edge of the injected fluid is uniform, asindicated by the circle 14. As the injection operation continues, thepressure due to surrounding injection wells tends to flatten the leadingedge pattern to the position shown at 16 in FIG. I and finally a cusplike pattern will [be produced as indicated at l8-in which the injectedfluid reaches one or more ofthe producing wells 12. When the injectedfluid is a gas it is necessary to seal off the breakthrough point sothat other portions of the formation may continue to be flush-ed out.However. this requires a determination of the depth at which thebreakthrough of the injected gas has occurred. Furthermore, if theinjected gas is hydrogen free or has a low hydrogen content. theconventional methods of detecting the breakthrough point which involvemeasurement of the temperature within the borehole to' determine thepoint at which cooling due to expansion of the entering gases occurs, ora density measurement to determine the point at which the gas mixes withthe borehole fluid, do not provide accurate results since the enteringgas must be capable of mixing readily with the hydrocarbon boreholefluid. The method of the present invention takes advantage of the factthat a neutron log is normally made at the time the well is drilled andthis neutron log is run at a certain sensitivity. The sensitivitysetting is usually part of the data given on the neutron log.Furthermore, the neutron log is particularly sensitive to the hydrogencontent of the rock formations surrounding the borehole, and isrelatively insensitive to the characteristics of the borehole fluid.Accordingly, the initial neutron log which is normally run at the timethe well is drilled provides an accurate measurement of the hydrogencontent of the rock formations adjacent the borehole wall beforesecondary recovery operations are started. However, if an initialneutron log is not available on the producing wells, such neutron logsare run in accordance with the present invention, prior to theinitiation of secondary recovery operations so that this informationwill be available and can be used in accordance with the presentinvention to detect the breakthrough pointof the injected gas into aproducing well in an accurate manner.

After a neutron log has been run in a producing well, :a gas having arelatively low hydrogen content as compared with the hydrocarbonproducts within the formations adjacent the producing well is injectedinto the surrounding formations. This gas injection step is normallyaccomplished by a centrally located injection well, as describedgenerally heretofore. When a breakthrough of the injected gas into theproducing well occurs a second neutron log is then run in the boreholein correlation with depth and the completed log is then correlated withthe initial neutron log to determine the point of entry of the injectedgas. However, in order to provide an accurate correlation, it isnecessary that the second neutron log be run at the same sensitivity asthe initial neutron log which was made prior to initiation of thesecondary recovery operations. This may be conveniently done by anarrangement such as indicated in FIG. 2 wherein a typical neutronlogging system is shown. Referring to this figure, the neutron loggingsystem may comprise a subsurface unit which is lowered by means ofthecable 22 into the borehole 24. Subsurface unit 20 comprises a detectingsystem 26 and a neutron source 28. The cable 22 suspends the logginginstrument in the well and contains the required conductorsforelectrically connecting the instrument with the surface apparatus. Thecable is wound on or unwound from drum 30 in raising or lowering theinstrument 20 to traverse the well. In making a neutron log of the well,the subsurface instrument 20 is raised within the well and neutrons fromthe source 28 irradiate the formation surrounding the borehole andradiations as influenced by the formations are detected by the detectingsystem 20. These radiations which are detected may either comprise slowneutrons or gamma rays of capture which result from the slowing down ofneutrons within the formation due to interaction with hydrogen atomstherein.

The resultant signal is sent to the surface through the cable 22 andthrough slip rings 32 and brushes 34 on the end of the drum, the signalmay be supplied to an amplifier 36 for amplification and processing, theoutput of the amplifier 36 being supplied to a rate meter 38 which inturn supplies this output to a suitable recorder 40. The recorder 40 isdriven through a transmission 42 by a measuring reel 44 over which thecable 22 is drawn so that recorder 40 moves in correlation with depth asthe subsurface instrument 20 traverses the well.

The elements described thus far are shown diagrammatically and it is tobe understood that the associated circuits and power supplies areprovided in a conventional manner. Amplifier 36 may be included in thesubsurface instrument, or amplification may be effected both on thesurface and in the subsurface instrument. It is also to be understoodthat the instrument housing will be constructed to withstand thepressures and mechanical and thermal abuses encountered in logging thewell and provides adequate space within it to house the necessaryapparatus and permit transmissionof radiation through it. 7

In a typical neutron logging system such as shown in FIG. 2, it iscustomary to provide an arrangement whereby the sensitivity of the logmay be varied sothat logs may be run at the same sensitivity andcompared to provide correlated information of the geological formationswhich have been logged. Such a sensitivity adjustment is indicateddiagrammatically inFIG. 2 as the potentiometer 46 which is shownconnected to the output of the rate meter 38, the potentiometer 46 beingnormally adjusted during an initial calibration procedure so that apredetermined number of counts per second in the detecting system 26provides a certain number of deflection units of the pen of the recorder40. A neutron calibrating system and sensitivity adjustment is shown,for example, in Scherbatskoy US. Pat. No. 3,038,076. However, it will beunderstood that any other suitable sensitivity adjusting arrangement maybe employed insofar as the present invention is concerned.

Considering now the manner in which the second neutron log is run inaccordance with the present invention to determine the breakthroughpoint of the injected gas in a producing well, the sensitivityadjustment potentiometer 46 is first adjusted so that the deflectionsensitivity of the recorder 40 is exactly the same as the deflectionsensitivity employed when the initial neutron log was run, either at thetime the well 7 was drilled or before secondary recovery operations areinitiated. After this sensitivity adjustment has been made, thesubsurface instrument 20 is then lowered within the well and a secondneutron log is made in cor relation with depth to permit determinationof the breakthrough or entry point of the injected gas. After the secondneutron log has been made, it is correlated with or compared to theinitial neutron log in the manner shown in FIG. 3 of the drawings.

Referring to this figure, the initial neutron log is shown in dottedlines as the curve 50 and the second neutron log, which is made afterthe breakthrough of the injected gas occurs, is shown in full lines asthe curve 52 in FIG. 3. These two curves are superimposed in FIG. 3 bycorrelating the depth measurement lines 54 of the two logs, it beingunderstood that the log is usually made from the bottom of the boreholeto the surface and hence the curves of FIG. 3 are shown with thegreatest depth at the bottom of the correlated graphs. It will be notedfrom FIG. 3 that at the lowest depth shown therein the two curves 50 and52 follow generally the same pattern indicating that the hydrogencontent in this portion of the formations has not changed due to thesecondary recovery operations. However at the point identified in FIG. 3as gas entry" it will be observed that the curve 52 is displacedsubstantially to the right as compared to the curve 50. Thisdisplacement of the curve 52 to the right corresponds to a markeddecrease in hydrogen content in the formations at that particular depth.Since the injected gas has been chosen to be of relatively low hydrogencontent it will be evident that the marked disparity between the twocurves 50 and 52 is due to the entry of the injected gas into theformations at the depth indicated in FIG. 3. In this connection it willbe understood that if the injected gas has a relatively high hydrogencontent, then the two curves 50 and 52 will not diverge appreciablysince the displacing injected gas functions to provide substantially thesame neutron logging signal as the hydrocarbon products which itdisplaces. Preferably, the injected gas employed in accordance with thepresent invention is either a hydrogen free gas such as carbon dioxide,or the gas of low hydrogen content such as propane, butane or the like.In any event, the injected gas should have a lower hydrogen content thanthe hydrocarbon products in the formations initially logged in orderthat the above-described divergence between the curves 50 and 52 will beachieved.

It will also be noted from FIG. 3 that the curves 50 and 52 continue tobe relatively widely displaced from one another all the way up to thetop of the portion of the log shown in FIG. 3, but then the curves ceaseto diverge appreciably. This is due to the fact that the logs shown inFlG. 3 were actually made in a well in which tubing extended down into aportion of the casing within the borehole. The depth to which the tubingextends is indicated as the point at which the curves come back togethersince the second log has decreased sensitivity when the radiations haveto pass through both the tubing and the casing. However, it will beunderstood that a suitable divergence between logs may be obtainedthrough both tubing and easing providing the sensitivity for both theinitial and second logs are increased correspondingly. It will also beunderstood that the method of the present invention may be used withincreased boreholes as well as wells where casing has been set, it beingonly necessary to correlate the sensitivity of the two logs so that theabove-discussed divergence effect will be achieved. In this connectionit will be noted that the sensitivity of the neutron logs 50 and 52 issuch that the curves are responsive to the; hydrogen content of thehydrocarbon products native to the rock formations as well as thecharacteristics of the borehole fluid. Accordingly, it is not necessaryfor the injected gas to be completely mixed with the borehole fluid tobe detected in accordance with the method of the present invention.

It will also be understood that the method of the present invention mayalso be used for tracer surveys in petroleum wells, by injecting a lowhydrogen content gas into the well and following movement of theinjected gas by means of the above-described neutron logging techniques.Thus, by comparing the neutron log made after injection of the gas withan initial neutron log made before gas injection .an indication can beobtained of the nature of the surrounding formations, in a mannersimilar to techniques conventionally employed with radioactive tracers,and the like.

While there have been illustrated and described various embodiments ofthe present invention, it will be apparent that various changes andmodifications thereof will occur to those skilled in the art. It isintended in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the presentinvention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. In a secondary recovery system of the type in which gas is injectedinto the formation and forces oil into one or more adjacent producingwells, the method of detecting the entry of the injected gas into one ofthe producing wells which comprises the steps of, injecting into theformations at a point removed from one of the producing wells a gashaving less hydrogen content than the hydrocarbon products native to theformations adjacent said one of the producing wells, obtaining aninitial neutron log of said producing well prior to the entry of saidinjected gas into said producing well, obtaining a second neutron log ofsaid producing well after the entry of said injected gas into saidproducing well, and comparing said initial neutron log and said secondneutron log to determine the point of entry of said injected gas.

2. The method of claim 1 wherein said point of entry of said injectedgas is determined in accordance with the displacement of said secondneutron log relative to said initial neutron log.

3. The method of claim 1, wherein the portions of the borehole in whichsaid injected gas is present correspond to relatively low hydrogencontent displacements of said second log relative to said initial log.

4. The method of claim 1, wherein said initial neutron log and saidsecond neutron log are both run at the same sensitivity.

5. The method of claim 1 wherein said injected gas is carbon dioxide.

6. The method of detecting the point of entry ofa gas into a producingwell, which comprises the steps of first measuring the hydrogen contentof the hydrocarbon products native to the formations surrounding saidwell in correlation with depth, then injecting a gas having lesshydrogen content than said hydrocarbon products into the formationssurrounding the well by introduction of said gas at a point removed fromsaid producing well, then measuring the hydrogen content of thehydrocarbon products within the well and surrounding formations incorrelation with depth after said gas has been injected, and comparingsaid first and second named measurements to determine the location ofthe point of entry of said gas into said well as indicated by areduction in the hydrogen content of said second measurement relative tosaid first measurement at the same depth.

1. IN A SECONDARY RECOVERY SYSTEM OF THE TYPE IN WHICH GAS IS INJECTEDINTO THE FORMATION AND FORCES OIL INTO ONE OR MORE ADJACENT PRODUCINGWELLS, THE METHOD OF DETECTING THE ENTRY OF THE INJECTED GAS INTO ONE OFTHE PRODUCING WELLS WHICH COMPRISES THE STEPS OF, INJECTING INTO THEFORMATIONS AT A POINT REMOVED FROM ONE OF THE PRODUCING WELLS A GASHAVING LESS HYDROGEN CONTENT THAN THE HYDROCARBON PRODUCTS NATIVE TO THEFORMATIONS ADJACENT SAID ONE OF THE PRODUCING WELLS, OBTAINING ANINITIAL NEUTRON LOG OF SAID PRODUCING WELL PRIOR TO THE ENTRY OF SAIDINJECTED GAS INTO SAID PRODUCING WELL, OBTAINING A SECOND NEUTRON LOG OFSAID PRODUCING WELL AFTER THE ENTRY OF SAID INJECTED GAS INTO SAIDPRODUCING WELL, AND COMPARING SAID INITIAL NEUTRON LOG AND SAID SECONDNEUTRON LOG TO DETERMINE THE POINT OF ENTRY OF SAID INJECTED GAS.
 2. Themethod of claim 1 wherein said point of entry of said injected gas isdetermined in accordance with the displacement of said second neutronlog relative to said initial neutron log.
 3. The method of claim 1,wherein the portions of the borehole in which said injected gas ispresent correspond to relatively low hydrogen content displacements ofsaid second log relative to said initial log.
 4. The method of claim 1,wherein said initial neutron log and said second neutron log are bothrun at the same sensitivity.
 5. The method of claim 1 wherein saidinjected gas is carbon dioxide.
 6. The method of detecting the point ofentry of a gas into a producing well, which comprises the steps of firstmeasuring the hydrogen content of the hydrocarbon products native to theformations surrounding said well in correlation with depth, theninjecting a gas having less hydrogen content than said hydrocarbonproducts into the formations surrounding the well by introduction ofsaid gas at a point removed from said producing well, then measuring thehydrogen content of the hydrocarbon products within the well andsurrounding formations in correlation with depth after said gas has beeninjected, and comparing said first and second named measurements todetermine the location of the point of entry of said gas into said wellas indicated by a reduction in the hydrogen content of said secondmeasurement relative to said first measurement at the same depth.